IMAGE-BASED 3D RECONSTRUCTION DATA AS AN ANALYSIS …...Commission II . KEY WORDS: Photogrammetry,...
Transcript of IMAGE-BASED 3D RECONSTRUCTION DATA AS AN ANALYSIS …...Commission II . KEY WORDS: Photogrammetry,...
IMAGE-BASED 3D RECONSTRUCTION DATA
AS AN ANALYSIS AND DOCUMENTATION TOOL FOR ARCHITECTS: THE CASE OF
PLAKA BRIDGE IN GREECE
Τ. Kouimtzogloua,*, E. K. Stathopouloub, P. Agrafiotisb and A. Georgopoulosb
a National Technical University of Athens, School of Architecture, Dep. IV Architectural Technology
Patision 42 st., 10682, Athens, Greece - [email protected] b National Technical University of Athens, School of Rural and Surveying Engineering, Lab. of Photogrammetry
Zografou Campus, Heroon Polytechniou 9 st., 15780, Zografou, Athens, Greece - (elliestath, pagraf, drag)@central.ntua.gr
Commission II
KEY WORDS: Photogrammetry, image-based 3D reconstruction, Architecture, Cultural Heritage
ABSTRACT:
Μodern advances in the field of image-based 3D reconstruction of complex architectures are valuable tools that may offer the
researchers great possibilities integrating the use of such procedures in their studies. In the same way that photogrammetry was a
well-known useful tool among the cultural heritage community for years, the state of the art reconstruction techniques generate
complete and easy to use 3D data, thus enabling engineers, architects and other cultural heritage experts to approach their case
studies in an exhaustive and efficient way. The generated data can be a valuable and accurate basis upon which further plans and
studies will be drafted. These and other aspects of the use of image-based 3D data for architectural studies are to be presented and
analysed in this paper, based on the experience gained from a specific case study, the Plaka Bridge. This historic structure is of
particular interest, as it was recently lost due to extreme weather conditions and serves as a strong proof that preventive actions are of
utmost importance in order to preserve our common past.
* Corresponding author
1. INTRODUCTION
Image-based 3D surveys have become a common invaluable
tool towards the documentation of our architectural heritage
(Remondino and El Hakim, 2006; Remondino 2011). They are
usually applied as standalone 3D recording process or
combined with conventional topographic surveys, laser
scanning or other ranging techniques using various mounting
platforms to capture the object of interest from all possible
viewing perspectives, such as poles, tripods, Unmanned Aerial
Vehicles etc.
Plaka bridge (Figure 1a) was an one-arch limestone bridge
situated in the Epirus region in northwest Greece that was
recently lost due to massive flash floods. The bridge, a declared
monument under protection by the Greek State, was built over
the river Arachthos. It is not exactly known when the first
version of the bridge was constructed, but after a collapse in
1860, the bridge was rebuilt in a location 100m south of the
initial position (current bridge location). Subsequently, it was
rebuilt in 1863 and 1866, after two more collapses.
It was considered as the widest single arched bridge of the
Balkans, with a ~40 meter diameter of the main arch and an
average height of 20 meters. Next to the main arch, there were
two smaller ones, 6 meters wide, the so-called relief arches. In
February 2015, the central section of the bridge’s central arch
collapsed due to the aforementioned extreme weather conditions
caused on by prolonged heavy rainfall (Figure 1b). Most of the
collapsed parts, especially some of the most massive ones, lie in
the river area near the abutments that remain intact.
(a)
(b)
Figure 1. The bridge (a) before and (b) after the collapse.
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-2/W3, 2017 3D Virtual Reconstruction and Visualization of Complex Architectures, 1–3 March 2017, Nafplio, Greece
This contribution has been peer-reviewed. doi:10.5194/isprs-archives-XLII-2-W3-391-2017
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1.1 Motivation
Due to the significance of the bridge and its landmark meaning
for the region, a restoration project was initiated by NTUA
immediately after the collapse and is still ongoing. The project
aims to investigate all scientific aspects needed for any possible
reconstruction of the monument and implies, thus, a
multidisciplinary approach was decided combining the expertise
of different cultural heritage professionals.
Within the project deliverables, along with the constructional
and structural analysis, a series of architectural plans
documenting Plaka Bridge were to be provided. In the
presented work, the geometric, architectural and structural data
acquisition, process and analysis are treated separately as in two
study stages: before and after the collapse. Section 2 describes
the related work in the literature, followed by detailed data
acquisition and processing steps in Section 3 and concluding
remarks in Section 4.
2. RELATED WORK
Numerous studies and applications have been carried out
related to the use of conventional photogrammetry and more
modern image-based techniques in the field of cultural heritage
geometric recording in order to produce 3D point clouds,
models or orthoimages and vector plans (Kersten, and
Lindstaedt, 2012; Koutsoudis et al., 2014; Martínez et al., 2013)
State of the art 3D reconstruction algorithms are broadly being
applied in the recent years, tested even under extreme viewing
conditions. For instance, retrieving depth information by a
sequence of unordered images, being acquired by various
sensors, platforms and viewpoints and under harsh illumination
and environment changes is considered achievable up to a
satisfactory degree (Snavely, 2006; Furukawa et al, 2010;
Agarwal et al., 2011; Shan et al., 2013).
Various projects related to image-based 3D reconstruction of
lost Cultural Heritage exist, past or still running, mainly
referring to assets exposed to conflict or war areas of the world.
In Grün et al., (2004) the results of the image-based 3D
reconstruction of the Great Buddha, the tallest representation of
a standing Buddha in the world are reported. There, the virtual
reconstruction of the statue that was demolished was achieved.
The work of Fangi and Wahbeh (2013) discusses the spherical
photogrammetry as a metric documentation technique applied in
the Minaret of the Umayyad Mosque of Aleppo, a UNESCO
heritage site of the ancient city of Aleppo, destroyed during the
current civil war. In Vincent et al., (2015) the Project Mosul
(Rekrei) is presented. This project seeks to digitally reconstruct
heritage that has been lost through war, conflict, natural disaster
or other means, and preserve the memory of that heritage using
digital preservation schemes. Finally, in Bonacchi et al., 2014 is
provided a brief introduction to MicroPasts, a web-enabled
crowd-sourcing and crowd-funding project whose overall goal
is to promote the collection and use of high quality research
data via institutional and community collaborations, both on-
and off-line, while the crowd-sourced image-based modelling is
still under development. It should be underlined that even
among these projects, multiple research challenges are to be
faced, with the geometric reliability and accuracy of the 3D
results being of the most important. Accurate metric
reconstruction enable the possible use of such results as
valuable source not only for visualization and dissemination
purposes, but also for the geometric documentation or
reconstruction of lost cultural heritage.
Our study was challenging in various aspects. Data from three
different previous studies and surveying campaigns held
respectively in 1984, 1995 and 2005 carried out with diverse
(mostly older or traditional) acquisition methods needed to be
accessed and combined with the recent data and findings. More
particularly, these existing studies needed to be evaluated and
combined among them, but most importantly with the recent
field surveying acquired data of the remaining parts of the
bridge after the collapse (2016).
Older survey plans, such as facades presented with outlines
and/or standard hatch patterns (Figure 2), were mostly
considered as incomplete in terms of information needed for an
architectural restoration study, as they provide few or
insufficient information about crucial morphological elements
of the monument, such as details of the stonework, the metal
and wood reinforcement systems, etc. (Interdisciplinary
Research Team of NTUA, 2016).
Figure 2. Plaka bridge South Façade plan (1984).
On the other hand, image-based 3D reconstruction using state
of the art Structure from Motion (SfM) and Multiple View
Stereo (MVS) algorithms played a crucial role in retrieving
several missing data parts not being recorded in the previous
studies and was thus proven to be an efficient tool providing
essential information for both processing stages.
As presented in Stathopoulou et al., (2015), crowdsourced
images have been used in order to create the 3D model of the
south façade of the bridge as well as orthorectified images of
the bridge before the collapse using image based 3D
reconstruction software (Figure 3).
Figure 3. 3D point cloud of the bridge using crowdsourced
images by (Stathopoulou et al., 2015).
These data have been combined with the field surveying study
of 1984 and the more recent architectural restoration study of
2005, in order to create a series of plans of the monument
showing the state before the collapse (Figure 4). This
combinatory approach, led to the production of more complete
and detailed architectural plans of all aspects of the structure,
thus, eliminating the majority of the deficiencies and
information gap of the previous studies.
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-2/W3, 2017 3D Virtual Reconstruction and Visualization of Complex Architectures, 1–3 March 2017, Nafplio, Greece
This contribution has been peer-reviewed. doi:10.5194/isprs-archives-XLII-2-W3-391-2017
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Figure 4. South façade plan of Plaka bridge before the collapse.
The current state of the bridge after the collapse was
documented both with conventional field surveying methods as
well as with image-based techniques.
3. DATA ACQUISITION AND PROCESSING
3.1 Data acquisition campaign of 2016
The campaign for the documentation of the current state with
image-based methods was only possible to be accomplished one
year after the collapse due to practical constraints. As a side
effect, small parts of the structure had been removed or drifted
by the river flow, while the massive arch parts were partially
immersed in the river and the pillars remain stood (Figures 2,
6). For the image based survey a DSLR Canon EOS Mark III
full frame camera was used acquiring a total of 826 images. The
water flow, the dense vegetation, the highly inclined riverside
along with the hazard of possible local collapses of the
remaining parts implied constraints while designing the image
network.
Therefore, multiple lens combinations of 25, 50 and 300mm
were used in order to completely cover the object areas
avoiding lack of information. Due to harsh illumination changes
(direct sunlight), various combinations of shutter speed, focal
length and ISO settings were implied. Ground Control Points
were also used distributed on the entire surface of the object as
markers or characteristic points for scaling and georeferencing.
3.2 Data processing
Agisoft Photoscan Pro® software was used for the image based
3D reconstruction in order to create a dense point cloud of the
object and a triangulated textured mesh was generated thereafter
(Figure 5). Each part of the bridge ruins was treated as an
individual entity during data acquisition as well as the
processing steps.
High quality orthorectified images of both facades of the
remains of the bridge were produced. These data were in turn
prepared for use in a similar way as in the ones referring to the
before the collapse state in order to produce accurate 2D vector
architectural drawings of the monument in a following stage of
the study (Figure 6). The 3D point cloud and the high resolution
textured mesh provide the opportunity to examine all the areas
of interest in detail, especially those of particular difficulty in
access and/or danger in approach and verify the in situ remarks.
(a)
(b)
Figure 5. 3D textured meshes clouds of the (a) west and (b) east
ruins.
Figure 6. South façade plan of Plaka Bridge after the collapse.
3.3 Data use and re-use
The field surveying data was used by the architects in charge in
the following ways:
The orthorectified images of the bridge were combined with the
field surveying data of the campaigns of 1984, 2005 and 2015
in CAD environment (Figure 7). Accurate elevation plans of the
north and south facades were created, depicting the outline of
the monument, the past and current pathology (Figure 8) -
cracks, fissures, detachments, deformations (Figure 9),
biological degradation, etc.- and the sizes and types of the
construction elements and building blocks.
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-2/W3, 2017 3D Virtual Reconstruction and Visualization of Complex Architectures, 1–3 March 2017, Nafplio, Greece
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393
Figure 7. Details of redesign and data gathering process of the
South façade before the collapse, by combining orthoimages
with the 1984 data. From top to bottom: 1984 plan, 1984 plan /
orthoimage, 1984 plan / orthoimage / 2D CAD drawing, 2D
CAD drawing.
The image-based generated 3D mesh was also used by
examining it after texture information removal. By using only
shadow and global illumination information in a Sun simulation
study, the 3D models present only geometric visual information.
As colour and texture information is omitted, details are more
easily observed and evaluated (Figure 10).
Figure 8. Pathology elevation plan of the South Façade before
the collapse.
Figure 9. Deformation study of west (left) and east (right)
sections of the remains of the Bridge based on the 3D mesh
geometry.
Figure 10. Untextured detail of the east pillar ruins: Larger and
minor cracks are easily noticed, as well as grout depth and stone
sizes and shapes differentiations.
Accurate sections of the object were produced in the areas of
interest automatically from the 3D mesh in the commercial 3D
modelling software 3D Studio Max (Figures 11, 12). The
resulting section outlines were exported in CAD format for
further manipulation. Façade detail elements were also
vectorized directly from the 3D mesh and were combined with
the aforementioned plan sections and other conventionally
drawn elements (Figure 13). New hybrid drawings were created,
incorporating the accuracy of the reconstruction and at the same
time trying to express the unique architectural and structural
character of the monument.
The same method was used to produce the main plan of the
bridge after the collapse. The model extracted data were
combined with the survey plan of 2005 (Papageorgiou G. and
A., 2005) and all the recent - after the collapse - field surveying
acquired data. The result was once more a hybrid plan that
maintained the original and newer digital data accuracy (Figure
14).
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-2/W3, 2017 3D Virtual Reconstruction and Visualization of Complex Architectures, 1–3 March 2017, Nafplio, Greece
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Figure 11. Section creation process directly from the 3D mesh
in 3ds Max commercial software.
Figure 12. Sections of the remaining west pillar of the Bridge
created directly from the 3D mesh.
Figure 13. Façade of the West Pillar ruins. 2D elevation plan
created form the 3D model, enhanced with CAD drawn details
Figure 14. Plaka Bridge, West pillar plan, after the collapse.
This almost automatic process ensures that the originally
recorded 3D data is not distorted or modified by the user, driven
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-2/W3, 2017 3D Virtual Reconstruction and Visualization of Complex Architectures, 1–3 March 2017, Nafplio, Greece
This contribution has been peer-reviewed. doi:10.5194/isprs-archives-XLII-2-W3-391-2017
395
by subjective interpretations of analog data or possible user
errors. Data alteration and degradation ceases to occur and in
this way the data re-use ensures the high fidelity of the source
material in every stage of the procedure. It is important to note
however, that in this way, the whole chain of data exchange and
re-use is founded on the quality and accuracy of the source
material. Any mistakes or deficiencies in the original data
recording will continue to re-emerge in all stages of the
procedure or might pass undetected leading to possible
assessment errors in the following stages by researchers of other
disciplines.
Meanwhile, the digital recording of spatial information, apart
from creating a solid base, ensures that future professionals and
researchers will have access both to the original digital source
material, as well as to re-used data products (3D models, 2D
vector drawings).
This foundation, if needed, can be enhanced with qualitative
additions that express specific or characteristic architectural and
constructional elements that can be always distinguished and
separated from the digital source material, according to the
needs of each study.
The image-based created 3D model of the bridge ruins was also
used to distinguish minor changes and deformations of the
remains of the monument (Figure 9). The high geometric
accuracy of the 3D model grants the opportunity to examine
with the utmost detail any potential changes in geometry and
deformations that would have otherwise been difficult to
identify or record with conventional surveying methods. Such a
detailed 3D model provides the opportunity to the researchers to
immerse deeply in the process and examine their subject from
all possible aspects, both space and scale wise.
Also, the exact geometric state of the bridge after the collapse
was crucial for the redesign of the main geometric
characteristics of the monument (main arch, secondary relief
arches, pillars of the bridge, etc.) in a precise way (Figure 15).
The geometry of the current state had to be combined with the
ideal original form of the monument (e.g. the double curvature
of the main arch) after the removal of the pathology
deformations.
Figure 15. 3D verification, in relation to the ruins geometry of
the original outline tracing of the main arch, by using the
image-based generated 3D model.
During this process it was obvious that the digital recreation of
the geometric form of the monument can only be defined in the
three-dimensional space. Two dimensional plans are indeed the
basis of the preliminary study, but their verification and
implementation can only be applied in 3D space. In this aspect,
the 3D information is the base upon which all the design data
was placed, granting the possibility to assemble a well-
documented geometric restoration proposal that combines and
takes into account: a. the current state of the monument, b. the
ideal initial design and c. the restoration constructional
guidelines.
4. CONCLUDING REMARKS
The work within the interdisciplinary team requires complex
data exchange procedures in order to define in an accurate way
all the geometric, architectural and constructional aspects of the
monument. Image based 3D reconstruction methods provided
the link between both past and current spatial data and historical
memory. Information that would otherwise have been lost, was
made available and accessible for further future use. The
acquired image data, apart from providing beneficial
information and conclusions, become invaluable and essential
tools for the evolution of the methodology of the documentation
and restoration process not only for the Plaka Bridge case study,
but also in a wider sense. This case study is considered a fine
example of interdisciplinary cooperation for the benefit of
cultural heritage.
ACKNOWLEDGEMENTS
The preliminary study of the restoration of Plaka Bridge was
accomplished by a multidisciplinary team consisted of
researchers coming from several departments of the National
Technical University of Athens. In this paper, only a small part
of the whole effort is presented. We would like to acknowledge
the contribution of the Schools of Rural and Surveying
Engineering, Architecture, Civil Engineering, Mining and
Metallurgical Engineering, Chemical Engineering and all the
other participants to the project within academia and private
sector. Acknowledgements are also given to the rest of the team
of the School of Architecture NTUA, M. Korres, H. Maistrou, I.
Efesiou, E. Tsakanika, M. Balodimou, M. Alexiou, for their
contribution and support. The support and cooperation of the
Greek Ministry of Culture and the Prefecture of Epirus is also
acknowledged. Additionally the contribution of the team of the
Laboratory of General Geodesy of the School of Rural and
Surveying Engineering, S. Dogouris, D. Tsinis and H. Perakis,
is acknowledged for the GCPs measurements. Finally, E.
Karakosta, B. Papanagiotou, N. Tragaris, Th. Chatzigeorgiou,
O. Arampatzi, S. Doggouris, D-D Mpalodimos, G.
Papageorgiou and A. Papageorgiou, and Bakalis D., are
acknowledged for providing the data and the results of the
surveys of 1984, 1995 and 2005.
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This contribution has been peer-reviewed. doi:10.5194/isprs-archives-XLII-2-W3-391-2017
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This contribution has been peer-reviewed. doi:10.5194/isprs-archives-XLII-2-W3-391-2017
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